This invention relates to laser systems, particularly but not exclusively for providing output profiles suitable for laser crystallisation of semiconductor films, such as crystallisation of amorphous silicon to form polycrystalline silicon in thin film devices. The invention also relates to apparatus and methods for crystallising a semiconductor film using such a laser system.
It is well known that the use of laser crystallisation processes can enable the manufacture of low-temperature poly-silicon devices, for example for displays or other circuits. However, the laser crystallisation process can give rise to poor yields, resulting from poor uniformity of the laser crystallisation process.
One factor contributing to this poor uniformity is the use of a simple so-called top-hat beam profile. This form of output is typical of known laser beam homogenisers. The top-hat profile has a substantially constant intensity over the width of the profile. Thus, when an amorphous silicon film is crystallised by scanning this beam across it, the amorphous silicon film is exposed to a substantially constant intensity. This intensity must fall within a very narrow range for the crystallisation process to be successful. This is because it is required that nearly all of the film is melted during the crystallisation process, but there should be no full melt-through of the amorphous silicon layer. During cooling, a portion of the layer which has experienced full melt-through will recrystallise into a fine-grained structure giving rise to reduced quality in the final product. The intensity may exceed the maximum value and thereby achieve full melt-through as a result of jitter in the output from the laser source.
Because the laser output is scanned across the sample being treated, an individual laser pulse which exceeds the maximum desired intensity will be the first laser shot applied to some areas of the substrate, but will be the last laser shot applied to other areas. The region of the substrate for which that pulse was the last shot will recrystallise into this fine-grained structure since no further laser heating is carried out.
The present invention provides a laser system comprising a laser source for producing a laser beam having a first intensity profile in one transverse direction, and a lens system for modifying the first intensity profile, the lens system comprising a plurality of lens elements adapted to divide the beam into a plurality of beamlets across the first intensity profile, at least one of the beamlets outputted by the lens system being inverted relative to the others, such that a desired intensity profile is generated at the output of the laser system. This lens system enables the beam generated by the laser source to be modified so that the output of the laser system is altered as required for laser crystallisation, addressing the problem identified above. This is achieved substantially without loss of beam energy or increase in fluence jitter.
In a preferred embodiment, the lens elements are arranged in two spaced arrays, each extending along the one direction, the arrays being adapted to invert the at least one beamlet relative to the others. Alternatively, the lens system may comprise two spaced arrays of primary lens elements, each extending along the one direction, with pairs of lens elements, one from each array, being arranged to invert a respective beamlet, wherein one or more additional lens elements are included to prevent inversion of the at least one beamlet or to provide a further inversion of the at least one beamlet. Thus, for example, additional lens elements may be inserted in an existing lens system to adapt it in accordance with the invention. Prevention of inversion rather than further inversion is preferred, as the latter may lead to greater light losses and divergence of the beamlets leaving the lens system.
The additional lens elements may comprise one or more pairs of secondary lens elements arranged between respective pairs of primary lens elements. Preferably, each of the secondary lens elements has a concave surface.
The at least one of the beamlets are preferably those located to one side of the centre of the first intensity profile. More particularly, where there is an even number of beamlets of substantially equal width, the at least one of the beamlets may be those in one half of the intensity profile. If there is an odd number of beamlets of substantially equal width, the at least one beamlets are those to one side of the central beamlet. The number of beamlets inverted may also be varied to suit particular requirements, thereby altering the overall impact of the inversions.
Preferably, the intensity profile outputted by the laser system is tapered over at least part of the width of the profile. This may be advantageous for reasons discussed below.
The laser source may comprise a pulsed source, generating a beam having a semi-Gaussian profile.
An optical filter may be provided also to modify the intensity profile generated at the output of the laser system. Such a filter is the subject of a co-pending U.S. patent application Ser. No. 09/643,486 filed Aug. 22, 2000, the contents of which are hereby incorporated herein by reference. The filter may comprise transmissive portions and opaque portions, the transmissive portions defining a repeating pattern with a pitch corresponding to the lens pitch of the primary lens elements.
The present invention also provides laser apparatus comprising a laser system of the invention, and means for scanning the width of the desired intensity profile relative to the surface of a sample to be laser treated. The scanning may be achieved by mounting the sample on a movable support.